The present invention relates to a hydrodynamic bearing, and more particularly, to a hydrodynamic bearing comprising a radial bearing portion and a thrust bearing portion. The present invention also relates to a spindle motor comprising the hydrodynamic bearing.
A small and light-weight spindle motor having a high rotation accuracy is used for a storage apparatus such as a hard disk, or a drive apparatus for a polygon mirror of a bar code scanner, etc. Since hydrodynamic gas bearings or hydrodynamic fluid (oil) bearings have simple structures and demonstrate stable bearing functions, it is known that these various types of hydrodynamic bearings are used for spindle motors. FIG. 7 shows an example of a hydrodynamic bearing disclosed in Japanese Patent Kokai (A) Publication No. 55918/1999. Referring to FIG. 7, a shaft 2 is fixed to a housing 1, and a rotor 3 is fitted on an outer circumferential surface of the shaft 2 such that the rotor 3 freely rotates. A stator 4 which forms an electro-magnet is attached to the outer circumferential surface of the shaft 2, and a rotor magnet 5 is attached to an inner circumferential surface of the rotor 3 which is faced with the stator 4. As a coil wound around the stator 4 is energized, drive force induced by attraction/repellence force is developed between the stator 4 and the rotor magnet 5, which in turn rotates the rotor 3 relative to the shaft 2 in the example shown in FIG. 7.
Radial hydrodynamic pressure generation elements 7 and 8 are attached, in a mutually opposed location, to the outer circumferential surface of the shaft 2 and the inner circumferential surface of the rotor 3, respectively, and relative rotation of these two generates a radial hydrodynamic pressure. Further, thrust hydrodynamic pressure generation elements 9 and 10 are attached, in a mutually opposed position, to the housing 1 and the rotor 3, respectively. A groove or grooves for generating a thrust hydrodynamic pressure are formed in either one of the mutually opposed surfaces 9 and 10, and a thrust hydrodynamic pressure is generated as the rotor 3 rotates. As a result, the rotor 3 is lifted up relative to the housing 1 and the rotor 3 rotates in a non-contact condition.
However, in the hydrodynamic bearing disclosed in Japanese Patent Kokai (A) Application No. 55918/1999, the thrust hydrodynamic pressure generation elements 9 and 10 relatively rotate in contact condition with each other until a sufficient thrust hydrodynamic pressure is generated after the rotor 3 starts to rotate from a halt state. Hence, there exists a problem that friction is created between the thrust hydrodynamic pressure generation elements 9 and 10 and the durability is accordingly deteriorated. The groove for generating a thrust hydrodynamic pressure usually has a spiral shape and it is formed by means of etching or laser engraving, and therefore, involves a high production cost problem.
To solve the problem of contact rotation as described above, Japanese Patent Kokai (A) Application 69715/1999 discloses a hydrodynamic bearing structured as shown in FIG. 8. In FIG. 8, the like elements as those in FIG. 7 bear the like reference numerals. A shaft 12 is fixed to a housing 11, and a hollow cylindrical rotor 13 with a blocked end on one side is fitted on an outer circumferential surface of the shaft 12. Drive force for a motor is obtained between a rotor magnet 5 fixed to an outer circumferential surface of the rotor 13 and a stator 4 disposed to the housing 11 facing the rotor magnet 5. As the rotor 13 rotates, a radial hydrodynamic pressure is generated between the outer circumferential surface of the shaft 12 and an inner circumferential surface of the rotor 13. A herringbone-shaped groove is formed on the outer circumferential surface of the shaft 12. As air introduced by the groove is guided to an upper portion of the rotor 13 having the blocked end and applies a pressure around this portion, the rotor 13 is lifted up. On the other hand, a ring-shaped thrust bearing member 14 is disposed to an outer peripheral portion of the rotor 13, while a thrust retaining member 15 is disposed to the housing 11, opposed with the thrust bearing member 14. As the rotor 13 is lifted up by the pressure of the introduced air, the thrust bearing member 14 moves close to the thrust retaining member 15. This develops a thrust hydrodynamic pressure between the two members which are brought close to each other, whereby the rotor 13 is maintained at a predetermined height at which the thrust hydrodynamic pressure balances with the pressure of the air described above and hence smoothly rotates in a non-contact condition.
Although the bearing structured as disclosed in Japanese Patent Kokai (A) Application No. 69715/1999 can avoid contact friction between the thrust hydrodynamic pressure generation elements 9 and 10 after the start of rotation, it is necessary to additionally dispose the thrust bearing member 14 and the thrust retaining member 15. This makes the structure of the bearing complex and increases the size and the weight.
Accordingly, an object of the present invention is to provide a hydrodynamic bearing having a small size and a light weight with low cost, and exhibiting a high rigidity despite a simple structure, which can solves the problem as described above associated with the prior art. An object of the present invention is also to provide a reliable spindle motor comprising such a hydrodynamic bearing.
According to the present invention, gas introduced at a radial bearing portion of a bearing is guided to a thrust bearing portion which is connected with the radial bearing portion, and the pressure of the introduced gas is used as a pressure for the thrust bearing portion. Hence, it is possible to obtain stable thrust force with a simple structure, without separately disposing a mechanism for generating a thrust hydrodynamic pressure. The present invention specifically includes the following.
That is, one aspect of the present invention relates to a hydrodynamic bearing comprising a radial bearing portion and a thrust bearing portion, which is characterized in that a groove or grooves capable of introducing fluid are formed on either one of surfaces opposed with each other at the radial bearing portion, and one end of the thrust bearing portion is connected with the radial bearing portion, and the other end of the thrust bearing portion is open to the outside atmosphere.
Another aspect of the present invention relates to a hydrodynamic bearing, wherein the groove or grooves exert a function of guiding the fluid introduced at the radial bearing portion to the thrust bearing portion connected with the radial bearing portion.
Yet another aspect of the present invention relates to a hydrodynamic bearing comprising a column-like shaft, and a hollow cylindrical sleeve whose one end is open to the outside and other end has a blocked surface, which sleeve is fitted on an outer circumferential surface of the shaft so as to accomplish relative rotation among the two, which is characterized in that in a radial bearing portion formed by the outer circumferential surface of the shaft and an inner circumferential surface of the sleeve which are faced with each other, a groove or grooves capable of introducing fluid are formed on either one of the outer circumferential surface of the shaft and the inner circumferential surface of the sleeve; in a thrust bearing portion formed by one end surface of the shaft in the axial direction and the blocked surface of the sleeve which are faced with each other, at least one opening capable of discharging the introduced fluid to outside atmosphere is formed either in the shaft or the sleeve; and the thrust bearing portion is formed contiguous to the radial bearing portion.
Yet another aspect of the present invention relates to a hydrodynamic bearing comprising a column-like shaft, and a hollow cylindrical sleeve whose one end is open and other end has a thrust surface which expands toward outside in the radius direction, which sleeve is fitted on an outer circumferential surface of the shaft so as to accomplish relative rotation among the two, which is characterized in that in a radial bearing portion formed by the outer circumferential surface of the shaft and an inner circumferential surface of the sleeve which are faced with each other, a groove or grooves capable of introducing fluid are formed on either one of the outer circumferential surface of the shaft and the inner circumferential surface of the sleeve; in a thrust bearing portion formed by the thrust surface of the sleeve and a thrust opposing surface fixed to one end of the shaft expanding toward outside in the radius direction, a releasing portion capable of discharging the introduced gas is formed at an outer periphery of the thrust bearing portion in the radius direction; and the thrust bearing portion is formed contiguous to the radial bearing portion.
Yet another aspect of the present invention relates to a hydrodynamic bearing, wherein the groove capable of introducing gas is a spiral groove inclined with respect to an axis of the hydrodynamic bearing.
Yet another aspect of the present invention relates to a hydrodynamic bearing wherein, the groove capable of introducing gas is a herringbone-shaped groove.
Yet another aspect of the present invention relates to a hydrodynamic bearing, wherein a groove-shaped asperity for generating a thrust hydrodynamic pressure is not formed on either surface which is faced with each other at the thrust bearing portion.
Yet another aspect of the present invention relates to a hydrodynamic bearing, wherein a ratio of surface area of the groove formed on either one of the surfaces opposed with each other at the radial bearing portion to the surface of the radial bearing portion which bears the groove is 40% or less.
Yet another aspect of the present invention relates to a hydrodynamic bearing, wherein the number of grooves formed in the radial bearing portion is six or smaller.
Yet another aspect of the present invention relates to a hydrodynamic bearing, wherein the shape of the groove formed in the radial bearing portion is asymmetric in such a manner that the depth in an upstream portion of an airflow which is the fore side in the direction of rotation is deep, and the depth in a downstream portion of the airflow which is the aft side in the direction of rotation is shallow.
Yet another aspect of the present invention relates to a hydrodynamic bearing, wherein either one or both of the shaft and the sleeve forming the bearing are made from ceramics.
Yet another aspect of the present invention relates to a hydrodynamic bearing, wherein the groove in the radial bearing portion for introducing fluid is formed on the outer circumferential surface of the shaft.
Still another aspect of the present invention relates to a spindle motor comprising any one of the hydrodynamic bearings described above.